The goals of the proposed research are to investigate the functional mechanism used by LytTR Regulatory Systems (LRS) to control gene expression and to explore their role in controlling a cell death pathway in Streptococcus mutans. LRS are a novel class of signal transduction system that consists of a LytTR family transcription regulator and putative membrane protein inhibitor that antagonizes the function of this regulator. The goals of this project will be achieved in two aims by: 1) determining the inhibitory control mechanism employed by two subtypes of LRS and 2) determining the hierarchical transcription control mechanism in the LRS cell death pathway. 1) To determine how LRS membrane proteins function as inhibitors of their cognate transcription regulator proteins, we will examine variety of potential posttranslational inhibitory control mechanisms and map the key residues in LRS proteins responsible for mediating the inhibition. Protein-protein interaction studies will determine whether LRS membrane proteins exert their function directly. 2) To determine how concurrent activation of the HdrRM and BrsRM LRS triggers a cell death pathway, we will test key aspects of a positive feedback cross-regulation model between LRS. Promoter binding studies will be correlated with transcriptional output responses.